Blueprint: Stoichiometry and Basic Chemistry Math

percent yield

% yield = (actual yield/theoretical yield) x 100

empirical formula

- states only the ratio of atoms in a compound. example: glucose = CH2O - value of empirical formula is identifying molecules with common atomic ratios and/or similar properties - limitation is that it tell us nothing about the total numbers or atoms or the overall weight

The Solvay Process is a reaction that uses sodium chloride and calcium carbonate to produce sodium carbonate and calcium chloride. Suppose a student wishes to obtain 2.5 moles of sodium chloride. How many grams of sodium chloride should the student measure out?

145g

The preparation of acetic acid from ethanol can be represented as follows: C2H5OH (aq) + O2 (g) → CH3COOH (aq) + H2O (l) If 92 grams of ethanol reacts with excess oxygen, how many moles of acetic acid are produced?

2 moles

A student prepares to complete a lab report covering decomposition reactions. The first reaction studied is the decomposition of ammonium dichromate: (NH4)2Cr2O7 (s) → Cr2O3 (aq) + H2O (l) + N2 (g) The student attempts to decompose 500 grams of ammonium dichromate but only produces 12 grams of N2 gas. What is the percent yield of N2? (Note: Molar mass of ammonium dichromate is approximately 250 g/mol.)

21%

A particular compound has the empirical formula C2H6ON. Which of the following options is closest to the percent composition by mass of nitrogen in this compound?

23%

Methane bubbles from the ocean floor and mixes with water, as represented by the following chemical equation: CH4 (g) + H2O (l) → CO (g) + 3H2 (g) A student mixes 60 mg methane with 36 mg water in a closed container. What is the theoretical yield of carbon monoxide?

56 mg CO

Methane bubbles from the ocean floor and mixes with water, as represented by the following chemical equation: CH4 (g) + H2O (l) → CO (g) + 3H2 (g) After mixing 60 mg methane with 36 mg water in a closed container, the student produces 42 mg of carbon monoxide. What is the percent yield in her experiment?

75%

A student prepares to complete a lab report covering decomposition reactions. The first reaction studied is the decomposition of ammonium dichromate: (NH4)2Cr2O7 (s) → Cr2O3 (aq) + H2O (l) + N2 (g) Theoretically, how many grams of nitrogen gas should be obtained from the decomposition of 750 grams of ammonium dichromate? (Note: Molar mass of ammonium dichromate is approximately 250 g/mol.)

84 g N2

molecular formula

A chemical formula that shows the number and type of each atom in a molecule, but not the arrangement of the atoms. example: glucose = C6H12O2

What is the empirical formula of a 92 g sample that contains roughly 52% carbon, 15% hydrogen, and 33% nitrogen by mass?

C2H7N

Which of the following is a possible molecular formula of a molecule with a molar mass of 120 g/mol and an empirical formula of CH2O?

C4H8O4: This answer satisfies both the empirical formula and the molar mass. This compound's molar mass is calculated as follows: (4 x 12 g/mol) + (8 x 1 g/mol) + (4 x 16 g/mol) = 120 g/mol. Therefore, we know the molecular formula must contain 4 moles of carbon and oxygen and 8 moles of hydrogen.

What is the molecular formula of a molecule with an empirical formula C3H7N and a molar mass of 174 g/mol?

C9H21N3

What is the empirical formula for a 75-g sample containing 30 g carbon, 5 g hydrogen, and 40 g oxygen?

CH2O

An unknown compound is found to contain 80% carbon and 20% hydrogen by mass. What is this compound's empirical formula?

CH3

An unknown compound is found to contain roughly 42% carbon, 10% hydrogen, and 48% nitrogen by mass. What is this compound's empirical formula?

CH3N

What is the molecular formula of an unknown sample that contains 92% carbon and 8% hydrogen by mass?

cannot be determined

What is the molecular formula of a sample weighing 56 g that is composed of 48 g carbon and 8 g hydrogen?

cannot be determined; We were not given the molar mass of the compound, and therefore we cannot determine the molecular formula. To find the empirical formula, convert the grams of each substance into moles. 48 g of carbon (~ 12 g/mol) is 4 moles and 8 g of hydrogen (~ 1 g/mol) is 8 moles. Thus, our ratio of carbon to hydrogen is 4:8 or 1:2. The empirical formula is CH2, but we cannot determine the molecular formula from the information given.

Copper(II) sulfide (CuS) and nitric acid (HNO3) reacts yielding copper(II) nitrate [Cu(NO3)2], sulfur (S), water (H2O), and nitrogen monoxide (NO). When 6 moles of copper(II) sulfide and 16 moles of nitric acid are mixed in a reaction vessel, what is the limiting reagent?

neither reagent is limiting

percent mass

relative amounts can be given in the form of percent mass

stoichiometry

technique used to calculate the amounts of reactants or products in a balanced chemical reaction; based on dimensional analysis

Methane bubbles from the ocean floor and mixes with water, as represented by the following chemical equation: CH4 (g) + H2O (l) → CO (g) + 3H2 (g) In a biology experiment, 30 mg of methane and 18 mg of water are mixed in a closed container. True or false: The limiting reagent in this experiment is H2O.

true

identifying limiting reagent

use moles, NOT grams for comparing amounts of different species because coefficients determine the number of moles that participate in a reaction.